Commentary on Meditations: B5:12

Here is a way to understand what sort of things the majority take to be 'goods'. If you think of the true goods there are wisdom, for example, self-control, justice, courage - with these in your mind you could not give any credence to the popular saying of  'too many goods to make room', because it will not apply. But bearing in mind what the majority see as goods you will hear and readily accept what the comic poet says as a fair comment. Even the majority can intuit this difference. Otherwise this saying would not both cause offence and rejection, while at the same time we take it as a telling and witty comment on wealth and the privileges of luxury and fame. Go on, then, and ask whether we should value and judge as goods those things which, when we have thought of them, would properly apply to their owner the saying, 'He is so rich, he has no room to shit.'

Virtue is the sole good.  If you want a detailed of explanation about this, see this Reddit post.  You can also read the entire chapter called Joy and Virtue in The Inner Citadel by Hadot.

In this passage, Marcus quite simply delineates what truly is good.  He lists out the four main virtues (wisdom, discipline, justice and courage).  Then he applies a popular saying to both these virtues and to indifferents. If I were to do the same thing

Therefore, when you insert an indifferent into the witty saying, it comes off as comical. Indeed, someone could have so many possessions, they have no room in which to take care of some personal business!

But if we used that same witty remark and inserted a virtue, such as courage, then it no longer seems comical. It doesn’t compute to state, “he is so courageous, he has no room to defecate.”

The key point which Marcus seems to be trying to make here is that there is a difference between an indifferent and a moral virtue.